client: add wit_client_init()

This function initialized empty wit_client structure with minimum
data needed for another functions (typically socket and wl_display)
wits: change wits versioning

Start versions from 0, not according to wayland.
server: rename functions

rename aliases for wit_display_process_request to be more intuitive
Also do appropriate changes in tests
client: add doc into header file

I used javadoc, because I've seen it used in wayland
server: add doc comments